The invention relates to circulating fluidized bed combustor apparatus and particularly to apparatus for cooling the ash of a fluidized bed. Circulating fluidized bed apparatus is being increasingly utilized for a wide variety of applications. The use of a circulating fluidized bed is particularly advantageous because of technological developments which have resulted in significant advances in both operating and fuel flexibility. While the present invention has primary application to a combustion process in a steam generating system, it will be understood that the present invention may also be used in a wide variety of fluidized bed apparatus.
Fluidized bed combustion apparatus can burn coal efficiently at temperatures low enough to avoid many of the problems of combustion in other modes. The term "fluidized bed" refers to the condition in which solid materials are given free flowing, fluid-like behavior. As a gas is passed upward through a bed of solid particles, the flow of gas produces forces which tend to separate the particles from one another. At low gas flows, the particles remain in contact with other solids and tend to resist movement. This condition is referred to as a fixed bed. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed is then deemed to be fluidized. The gas cushion between the solids allows the particles to move freely, giving the bed a liquid-like characteristic.
Fluidized bed combustion makes possible the burning of fuels having such a high concentration of ash, sulfur, and nitrogen that they would ordinarily be deemed unsuitable. By the use of this process it is possible, at least in most cases, to avoid the need for gas scrubbers while still meeting emissions requirements. In fluidized bed combustion, the fuel is burned in a bed of hot incombustible particles suspended by an upward flow of fluidizing gas. Typically the fuel is a solid such as coal, although liquid and gaseous fuels can be readily used.
The fluidizing gas is generally combustion air or the gaseous products of combustion. Two main types of fluidized bed combustion systems are (1) bubbling fluid bed (BFB) in which the air in excess of that required to fluidize the bed passes through the bed in the form of bubbles. The bubbling fluid bed is further characterized by modest bed solids mixing rate and relatively low solids entrainment in the flue gas and (2) circulating fluid bed (CFB) which is characterized by higher velocities and finer bed particle sizes. In such systems the fluid bed surface becomes diffused as solids entrainment increases, such that there is no longer a defined bed height. Circulating fluid bed systems have a high rate of material circulating from the combustor to the particle recycle system and back to the combustor. Characteristics of apparatus of this general type are further described in the publication Combustion Fossil Power, edited by Joseph G. Singer, P.E. and published by Combustion Engineering, Inc.; a subsidiary of Asea Brown Boveri, 1000 Prospect Hill Road, Windsor, Conn. 06095 (1991 edition).
In a conventional circulating fluidized-bed steam generator crushed fuel and sorbent are fed mechanically or pneumatically to the lower portion of a combustor. Primary air is supplied to the bottom of the combustor through an air distributor, with secondary air fed through air ports at one or more elevations in the lower part of the combustor. Combustion takes place throughout the combustor, which is filled with fluidized bed material. Flue gases and entrained solids leave the combustor and enter one or more cyclones where the larger solids are separated and fall to a seal pot. From the seal pot, the solids are recycled to the combustor. Optionally, some solids may be diverted through a plug valve to an external fluidized-bed heat exchanger (FBHE) and back to the combustor. In the FBHE, tube bundles absorb heat from the fluidized solids.
The present invention has application to any fluidized bed apparatus, however, it has particular application to circulating fluid bed boilers operating with a fuel that produces more than the usual amount of ash. Such fuels may be referred to as high ash fuels. A high ash fuel is a fuel having an ash that weighs 35% or more of the weight of the fuel. (Low ash fuels typically do not require a fluid bed ash cooler although some may be cooled with cooling apparatus such as a screw cooler. Screw coolers have a jacketed sleeve around a helix that it is rotated to move solid matter axially within the sleeve.) The ash produced in the fluid bed includes both the backpass ash and the bottom ash. It is essential that the temperature of the ash leaving the combustor be cooled so that the ash does not damage or destroy the conveying equipment.
The bottom ash should be cooled from combustor temperature to below 500 degrees Fahrenheit before entering the bottom ash conveying system. When a high ash fuel is used, the heat in the bottom ash stream may represent a significant percentage of boiler heat input. Consequently, it can be desirable to recover this heat. Fluidized bed ash coolers are generally used for this purpose. The fluidized bed ash cooler has a bubbling fluidized bed heat exchanger identical in design to the fluidized bed heat exchanger. Cooling coils immersed in the bed cool the ash and transfer heat to condensate or boiler feed water. Ash flow from the combustor 10 to the fluidized bed ash cooler 34 is optionally controlled by a cone valve as with the fluidized bed heat exchanger. However, a V-port or any control valve may be used to control the flow of ash into the ash cooler. Cooled ash from the ash cooler passes to the bottom ash handling system for transport to storage. This is usually a mechanical system consisting of flight conveyor's, although a pressured pneumatic system can also be used. Alternately, a mechanical system can be used to transport bottom ash to an intermediate hopper, from which a pneumatic system conveys the material to storage.
It is the usual in the prior art to position the outlet of the ash cooler above the bottom surface of the ash cooler. In other words, the outlet is at the end of a pipe which is raised above the bottom of the ash cooler so that some ash always remains in the ash cooler. This construction is satisfactory for many applications. However, for some applications this construction complicates the removal (from the ash cooler) of large particles that have not been fluidized can not easily be removed from the ash cooler.
The prior art coolers have traditionally been provided with a horizontal floor and typically been provided with a weir. The weir constrains the ash within the ash cooler. A disadvantage of such constructions is that the lighter particles move to the upper surface and the heavier particles move to the bottom. The lighter particles will flow over the weir and exit the ash cooler. The heavier particles have to be removed separately.
It is also usual in the prior art to provide an ash cooler that is substantially square as viewed from above. This construction has now been found to limit heat transfer. More specifically, ash entering a generally square ash cooler typically can flow to an outlet located at one side thereof without having substantial contact with heat exchange surfaces that may extend over substantially the entire floor of the ash cooler. This is unsatisfactory from a thermodynamic standpoint.